Display screen or portion thereof with a graphical user interface for an on-board computer



FIG. 1 is a front view of first embodiment of the graphical user interface for an on-board computer, or a portion thereof showing our new design; and

FIG. 2 is a front view of second embodiment of the graphical user interface for an on-board computer, or a portion thereof showing our new design.

Broken lines around the Figures show a display screen or portion thereof and form no part of the claimed design. Other broken lines show environmental features and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface for an on-board computer, as shown and described. 